Evaluation of freeze-dried Daratumumab immunoconjugate kits for Lutetium-177 radiolabeling

Monoclonal antibodies (mAbs) are highly therapeutically relevant drugs but face challenges in radiopharmacy as radionuclide carriers due to their complex structures, which can lead to degradation. Freeze-drying is commonly used to enhance their stability, especially in ready-to-use kits for radiolabeling. The use of various excipients can be beneficial, but their application in mAb based kits is limited due to specific radiolabeling requirements. Our study focused on two main aspects: optimization of immunoconjugate production and the excipients used to formulate the kit for radiolabeling. We used Daratumumab, an anti-CD38 human mAb, as a model for producing immunoconjugates. The conjugation was performed with a DOTA-NHS chelator and Lutetium-177 as a radionuclide for labeling. Five different formulations (1 mg/mL) with 0.1 M or 0.01 M phosphate buffer, mannitol and sucrose in various ratios and polysorbate 20 in the last formulation were prepared. The designed freeze-drying method includes freezing at -40°C, an annealing step to -15°C,
and again freezing to -40°C for a total of 6.5 h. During primary drying, the temperature was set to -10°C under vacuum (0.12 mBar) for 18 h, followed by secondary drying up to 25°C for 8 h. The reconstitution time, visual appearance, residual moisture (Karl–Fisher titration), and protein stability (size exclusion chromatography, SE-HPLC) were established as quality control attributes. Formulations containing saline exhibited lower residual moisture (< 3%) compared to water-based (~ 5%). The results from SE-HPLC showed higher purity (98.9–99.8%) and fewer aggregates in the formulations with water or saline compared to those with phosphate buffer pH 8.0 (93.9–95%). This indicates that a lower pH and optimized freeze-drying protocol may be preferable for Daratumumab immunoconjugates. Further
studies will focus on comparing the outcomes of non- radioactive tests, specifically evaluating radiochemical yield and purity after labeling.